The engine uses a deckless block, so there is a lot of the gasket exposed to coolant.  Over time the metal outer cracks, lets coolant into the matrix and rots the fire rings from the coolant side.

Replacement isn't a major job - just strip to where you can lift the head using one of the down bars (sweeps) as a lift point, clean up , replace and reassemble.

We have just replaced another one that was installed 2017, which is the third replacement.

The last one ran on green coolant, previous ones not so much.  And it seems more time related that hours of operation - this is a ranch use dozer.